Method of heat treating steel strip

ABSTRACT

THIS DISCLOSURE RELATES TO A METHOD FOR HEAT TREATING WORK HARDENED STEEL STRIP WHICH HAS BEEN COLD REDUCED 40 TO 80 PERCENT AND CONTAINS FROM 0.04 TO 0.1 PERCENT CARBON. THE STRIP IS RAPIDLY HEATED TO A TEMPERATURE TO WHICH AUSTENITE AND FERRITE EXIST UNDER EQUILIBRIUM CONDITIONS SO THAT IT RETAINS APPRECIABLE WORK STRESSES WHEN IT REACHES TEMPERATURE. THE STRIP IS THEN MAINTAINED AT SUCH TEMPERATURE UNTIL THE WORK STRESS IS RELIEVED, AND THEN IS HEATED TO A SECOND TEMPERATURE AT WHICH ONLY AUSTENITE EXISTS UNDER EQUILIBRIUM CONDITIONS. THE STRIP IS MAINTAINED AT THE SECOND TEMPERATURE UNTIL UNIFORMITY OF GRAIN SIZE IS ACHIEVED, AFTER WHICH THE STRIP IS COOLED.

United States Patent 3,711,342 METHOD OF HEAT TREATING STEEL STRIP Orville E. Cullen, 2728 Jodore Ave. 43606, and Joseph A. Lincoln, 3821 Driftwood Road 43614, both of Toledo, Ohio No Drawing. Continuation-impart of application Ser. No. 487,603, Sept. 15, 1965. This application May 23, 1969, Ser. No. 836,193

Int. Cl. C21d 1/26 US. Cl. 148-134 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to a method for heat treating work hardened steel strip which has been cold reduced 40 to 80 percent and contains from 0.04 to 0.1 percent carbon. The strip is rapidly heated to a temperature to which austenite and ferrite exist under equilibrium conditions so that it retains appreciable work stresses when it reaches temperature. The strip is then maintained at such temperature until the work stress is relieved, and then is heated to a second temperature at which only austenite exists under'equilibrium conditions. The strip is maintained at the second temperature until uniformity of grain size is achieved, after which the strip is cooled.

This application is a continuation-in-part of US. patent application Ser. No. 487,603, filed Sept. 15, 1965, now abandoned.

This invention relates to a method for heat treating steel strip and, more particularly, to a method for rapidly heat treating work hardened, low carbon, steel strip.

Low carbon steel strip containing from about 0.041 to about 0.1 percent of carbon is used commercially in, for example, the deep drawing of automobile bodies and in the production of galvanized steel. Generally, the s eel strip is hot rolled, pickled, and cold worked. The cold working reduces the final thickness of the strip by approximately 40 to 80 percent and the steel strip has a thickness after cold working of, for example, between 0.016 to 0.080 inch. The effect of the cold reduction is to harden the strip and to distort the grain structure. After cold reduction steel strip normally has a Rockwell B hardness in excess of 80. Consequently, for most applications it is necessary to anneal the steel strip after cold working to soften the strip and also to restore the desired grain structure.

In prior art annealing cycles, the steel strip was heated slowly to a temperature high enough to cause recrystallization, normally to a temperature of from between 1200 to 1300 F., and then cooled. The working stresses resulting from the cold working were relieved during this annealing step and uniform grain size was achieved. Under prior art methods, a batch type annealing step often took approximately 30 hours. Continuous type annealing methum phase as shown on the iron-carbon diagram. Similarly, the term upper critical temperature shall mean the criticalternperature between the ferrite plus austenite equilibrium phase and the austenite equilibrium phase. The steel strip which is processed by the method of the present I invention contains from about 0.04 to about 0.1 percent ods are known in the prior art, however, even though the 1 time required for annealing was considerably shorter than batch type operations, the processed deep draw steel strip had physical characteristics which rendered the strip unsuitable for the production of, for example, automobile bodies.

Under the method of the instant invention, the annealing step is reduced in time to approximately 10 minutes, or even less. Furthermore, steel strip processed by the present method has hardness characteristics comparable to strip processed in a 30-hour batch type annealing operatlon.

Where used in this specification and in the appended claims, the term lower critical temperature shall mean the critical temperature between the ferrite and cementite equilibrium phase and the ferrite plus austenite equi1ibriof carbon. Accordingly, the lower critical temperature in this range is approximately 1333 F. and the upper critical temperature is approximately 1630 F.

It is the primary object of the invention to provide a method for rapidly heat treating low carbon steel strip.

It is another object of the invention to provide a methtreated steel strip has a uniform grain structure of a desirable size.

It is another object of the invention to provide a method for annealing low carbon steel strip wherein the treated strip has a uniform grain structure of a desirable size.

' It is a further object of the invention to provide a method for continuously annealing low carbon strip steel whereby such method may be incorporated in a continuous galvanizing line.

It is a still further object of the instant invention to provide a method for continuously annealing low carbon steel strip, wherein deep drawing steel processed by the method has mechanical characteristics at least equivalent to deep drawing strip produced by known long time cycle batch annealing methods.

Briefly, the present invention relates to a method for heat treating work hardened steel strip containing from about 0.04 to about 0.1 percent of carbon. The method comprises heating the strip rapidly to a temperature at which austenite and ferrite exist under equilibrium conditions. The first heating step is performed at a sufficiently rapid rate that the strip, when it reaches such temperature, retains appreciable work stresses. The strip is then maintained at such temperature until the work stress is substantially completely relieved. The strip is then heated to a temperature T1 at which only austenite exists under equilibrium conditions. Next, the strip is maintained at the temperature T1 until substantial uniformity of grain size is achieved and, finally the strip is cooled.

A preferred time cycle, under the method of the present invention, is shown below and identified as Time Cycle A. It should be expressly understood that the present invention is not limited to the particular disclosure of the example.

TIME CYCLE A Equals 8.6 min.

Chart I, listed below, shows the average physical properties of several hundred test specimens of steel'strip which were heat treated on Time Cycle A.

CHART I.-TEST DATA [Average properties, standard 0.5 ASIM tensile specimens of low carbon 7 steel strip heat treated on time cycle A Longi- Transtudinal verse (A) Tensile strength (9st.) 47, 787 49, 25 (B) 0.2% olfset yield (p.s.i.) 38, 482 (O) Tensile/yield ratio (A/B) 1. 28 (D) Elongation in 2 (percent 33. 36 (E) Hardness (RB) 53. 69 54.54

It should benoted that the results shown in Chart I were obtained prior to a skin pass. It has been found that a 1 percent to 2 percent skin pass will lower the yield point approximately 3,000 p.s.i.

..The grain size of continuously processed strip, heat treated on Time Cycle A, is No. 7 or No. 8 as compared with No. Sand No. 6 grain size obtained by prior art batch annealing methods. The fine, uniform grain structure is highly desirable for improved surface finishes.

It has also been found (see Chart 1) that the longitudinal properties and the transverse properties of steel strip, processed by the present method, are very similar. This is another advantage of the present method compared with prior art batch annealing'methods.

Referring to Time Cycle A and Chart I, it will be observed that steel strip heat treated by the present method has physical properties suitable for use in deep drawing operations. Furthermore, the method is capable of being performed in a time cycle of less than 10 minutes.

It has been found that it is preferable to use rimmed steel strips, when practicing the method of the present invention, rather than, for example aluminum killed steel strip.

Prior to the first step of the present method, the steel strip has been Work hardened which reduces the hot rolled thickness by, for example, 40 to 80 percent. The work hardening results in a steel strip having a non-uniform grain structure and also produces residual stresses within 4 does not affect the micro-structure of the strip after such uniform grain size has been achieved.

The steel strip is then cooled. It has been found that excessively fast cooling should be avoided. Referring to Time Cycle A, in the preferred embodiment, the strip is first cooled to 850 F. during a time period of-70 seconds. Next, the strip is cooled to 650 F. in a period of 80 seconds and finally, the strip is cooled to 200 F. in a period of 150 seconds. i

It has been found that if the strip is cooled at too fast a rate, the strip tends to age harden at ambient temperatures. Possibly, the age hardening is caused by the entrapment of excessive carbon on other elements such as nithe, strip. For reasons not clearly understood, with respect to the first heating step of the present method, it has been found that it is necessary rapidly to heat the strip to a temperature which falls within theaustenite and ferrite phase. Such heating must be at a suificiently rapid rate that, when the steel strip reaches that temperature, it retains an appreciable amount of the residual stresses resulting from the work hardening. Referring to preferred Time CycleA above, the steel strip is heated to 1550 F. during a period of 75 seconds. The strip is then maintained at the 1550" F. temperature until the work stresses are substantially completely relieved. In Time Cycle A this dwell time or hold time is a period of seconds. During this dwell time recrystallization occurs and, as previously mentioned, the work stresses are relieved.

It has been found that if the first heating step, namely the initial heating to 1550 F., is extended to more than 75 seconds, whereby a larger portion of the work stresses are relieved during the temperature ascent, it is necessary to dwell at 1550" F. for a longer period of time to relieve the remaining work stresses. Apparently, the retaining of appreciably work stresses until the strip reaches the dwell temperature enable relatively slow nucleation, relative to the rate of grain growth, with the result that the remaining work stresses are relieved without causing an undesirably fine grained structure. i

It has also been found that the first dwell temperature, which is above the lower critical temperature and within the range where both ferrite and austenite exist under equilibrium conditions should be such as to insure a relathe grain size. When the temperature T1 is 1750' F. a

uniform fine grain (7 and 8 grain size) is obtained. The steel strip is maintained at the-temperature Tl until substantial uniformity of grain size is achieved. Uniformity of grain size may be attained with a hold period asfsmall as 2' seconds, but may require up'to seconds. I-t'has been found that additional dwell time at the temperature T l trogen in the ferrite during an extremely rapid cooling cycle. The carbon supersaturates the ferrite so that even at ambient temperatures the strip is not in equilibrium. As the strip changes to an equilibrium condition it undergoes age hardening.

*In another embodiment of the present method, cooling is retarded after the strip has been cooled to 850 F. .In the embodiment the strip is cooled between 850 F. and 650 F. at the rate of 10 F. per minute. Lengthening the time period for this particular step resulted in a significant improvement in mechanical properties.

Chart II, listed below, shows the physical properties of test specimens of steel strip which were heat treated on revised Time Cycle A.

CHART II.TEST DATA [Average properties, standard 0.5 AS'IM tensile specimens of low carbon steel strip heat treated on revised time cycle A] (F) Estimate yield after skin palm As previously mentioned, the first step of the method of the invention comprises heating the strip rapidly to a temperature at which both austenite and ferrite exist under eqiiilibrium conditions. In the optimum embodiment (see Time Cycle A) this temperature is 1550 F however, opti um results can be achieved when this temperature va ies between 1540 F. and 1550" F., and excellent resu ts are achieved so long as the temperature is between 1500 F. and 1600 F. Preferably, the first heating step is accomplished in approximately seconds or less to obtain otimum results, however, satisfactory results are obtained if the first heating step is accomplished in not more than about 5 minutes.

After the first heating step to the temperature which falls within the range where austenite and ferrite exist under equilibrium conditions, the strip is maintained at that temperature until the work stress is substantially completely relieved. Normally, this dwell time is 30 seconds, however, the dwell time varies with the duration of the first heating step. Dwell times as short as 15 seconds have been found to be satisfactory in some instances. Holding the strip at this temperature after the work stresses have been relieved does not affect the characteristics of the processed strip, but dwell times of excessive duration, of course, affect the production rate.

In the second heating step, the steel strip is heated to a temperature T1 at which only austenite exists under equilibrium conditions. Temperature T1 is above the upper critical temperature and its value is determined by the grain size desired in the processed strip. It has been found that to obtain between a number 6 and a number 8 grain size the temperature T1 sh'ould range between 1700" F. and 1850" F.

I The steel strip is held at the temperature T1 until a uniform grain size is achieved. Normally the holding time is at least 60 seconds. After uniform grainsize is achieved, additional dwell time at the temperature Tl does not adversely affect the micro-structure of the steel strip, however, it again increases the processing times. After being held at the temperature T1 the steel strip is cooled. Normally, the cooling step is not as critical as the first heating step. However, it has been found that the cooling rates should preferably be not greater than 770 F. per minute between the temperature T1 and 850 F.; not greater than 150 F. per minute between 850 F. and 650 F.; and not greater than 100 F. per minute between 650 F. and 200 F. Furthermore, improved mechanical properties may be achieved if the cooling rate is lowered to F. per minute between 850 F. and 650 F.

Deep draw steel strip, which has been processed according to the present method, is used in large quantities by the automotive industry. The present method may also be incorporated in continuous operations such as a continuous galvanizing line.

While the present invention has been described with respect to specific embodiments thereof, it should be expressly understood that these embodiments are for the purpose of description and numerous modifications fall within the scope of the appended claims. The method of the instant invention, while it is very suitable to a continuous heat treating process, it is not restricted toithis type of operation, but it is also readily adaptable to semicontinuous annealing or to batch annealing.

We claim: 1

1. A method for heat treating 40 to 80 percent cold work reduced hardened steel strip containing from about 0.04 to about 0.1 percent of carbon, said method cpmprising: heating the strip rapidly in not more than five minutes to a temperature between 1500 F. and 1600 F. at which both austenite and ferrite exist under equilibrium conditions, such heating being at a sufiiciently rapid rate that the strip, when it reaches such temperature, retains appreciable work stress, maintaining the strip at su h a temperature until the work stress is substantially completely relieved, heating the strip to a temperature Tl between 1700 F. and 1850 F. at which only austenite exists under equilibrium conditions, maintaining the strip at the temperature T1 in excess of approximately 60 seconds, and cooling the strip.

2. A method for annealing 40 to 80 percent cold work reduced hardened steel strip containing from about (5.04 to about 0.1 percent carbon, said method comprising: heating the strip in not more than five minutes to a rst temperature between 1333 F the lower critical temperature, and 1630 F., the upper critical temperature, maintaining the strip at such first temperature for a period of not more than 30 seconds until the work stresses of the strip are substantially relieved, heating the strip to a second temperature which is greater than the upper critical temperature but less than 1850 F., maintaining the strip at such second temperature in excess of approximately 60 seconds, and cooling the strip.

3. A method for heat treating work hardened steel strip which has been cold reduced 40 to 80 percent containing from about 0.04 to about 0.1 percent of carbon, said method comprising: heating the strip rapidly to a temperature from about 1500 F. to 1600 F. in not more than about five minutes, such heating being at a sufliciently rapid rate that the strip, when it reaches such temperature, retains appreciable Work stress, maintaining the strip at such a temperature for not more than 30 seconds to allow the work stress to be substantially completely relieved, heating the strip to a temperature T1 between 1700 'F. and 1850 F. at which only austenite exists under equilibrium conditions, maintaining the strip at the temperature T1 up to approximately 60 seconds, and cooling the strip.

4. A method for heat treating work hardened steel strip which has been cold reduced 40 to 80 percent containing from about 0.04 to about 0.1 percent of carbon, said method comprising: heating the strip rapidly in not more than five minutes to a temperature from about 1525 F. to about 1575 F., such heating being at a sutficiently rapid rate that the strip, when it reaches such temperature, retains appreciable Work stress, maintaining the strip at such a temperature until the work stress is substantially completely relieved, heating the strip to a temperature T1 between 1700 F. and 1850 F. at which only austenite exists under equilibrium conditions, maintaining the strip at the temperature T1 in excess of approximately 60 seconds, and cooling the strip at a rate not greater than 770 F. per minute between temperature T1 and 850 F., not greater than 150 F. per minute between 850 F. and 650 F. and not greater than 100 F. per minute between 650 F. and 200 F.

5. A method according to claim 4 wherein the first heating step is accomplished in not more than about 2 minutes.

6. A method according to claim 4 wherein the strip is at the temperature T1 for approximately 60 seconds before cooling is commenced.

7. A method for heat treating 40 to percent cold work reduced hardened steel strip containing from about 0.04 to about 0.1 percent of carbon, said method comprising: heating the strip rapidly in approximately 75 seconds to a temperature from about 1540 F. to about 1560 F., such heating being at a sufficiently rapid rate that the strip, when it reaches such temperature, retains appreciable Work stress, maintaining the strip at such a temperature for approximately 30 seconds to allow the work stress to be substantially completely relieved, heating the strip to a temperature T1 between 1700 F. and 1850 F. at which only austenite exists under equilibrium conditions, maintaining the strip at the temperature T1 in excess of approximately 60 seconds, and cooling the strip.

8. A method according to claim 7 wherein the cooling rate between the temperature 850 F. and 650 F. is not greater than 10 F. per minute.

9. A method according to claim 7 wherein the cooling rate between the temperature T1 and 850 F. is not greater than 770 F. per minute between 850 F. and 650 F. is not greater than 150 F. per minute, and between 650 F. and 200 F. is not greater than F. per minute.

10. A method for heat treating work hardened steel strip which has been cold reduced 40 to 80 percent containing from about 0.04 to about 0.1 percent of carbon, said method comprising heating the strip within a two minute period to a temperature from about 1540 F. to about 1560 F., whereby when the strip reaches such temperature it retains appreciable work stresses, maintaining the strip at such temperature until the work stresses are substantially relieved, heating the strip to a temperature T1 between 1750 F. and 1800 F., maintaining the strip at the temperature T1 for at least 60 seconds, cooling the strip from the temperature T1 to 850 F. at a rate not greater than 770 F. per minute, cooling the strip from 850 F. to 650 F. at a rate not greater than F. per minute, and cooling the strip from 650 F. to 200 F. at a rate not greater than 100 F. per minute.

References Cited UNITED STATES PATENTS 1,493,140 5/1924 Zimmerman 148-434 1,581,269 4/1926 Kelley 148-12 3,099,592 7/1963 Garber 148-12.3 X

FOREIGN PATENTS 487,844 9/1936 Great Britain 14812.3

OTHER REFERENCES Edwards et al.: Cold Rolling and Annealling, Blast Furnace and Steel Plant, July 1936, pp. 613 and 617.

Edwards et. al.: Rolling and Annealing of Steel Sheet, Metal Progress, pp. 78 and 84, June 1936.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. l4812, 12.3, 143 

